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I had a new Lennox Heat Pump and variable speed air handler installed in 1999, I think it's 4.5 tons and it has 2 zones. Recently while researching HVAC information at work for our line office expansion I realized how undersized my duct work is. I would like to come up with the best course of action then when I call a couple HVAC contractors in I can specify exactly what I want. I have no intention of doing this retrofit myself.

Basically I had a large home addition added in 1999, I had a 2 year old 2.5 ton Trane XL1200 heat pump for the original house that did an excellant job, when I added the addition I let the HVAC man talk me into scrapping the old system and adding a larger 4.5 ton Lennox unit to serve the whole house. I initially just wanted to add a second furnace and heat pump for the addition.

He took the old air handler out that was in the center of the supply trunk run, had about 20 feet of supply on each side of the AH. He then added the new unit at the far end of the house in the garage and tied the main trunk from the new AH into the end of the old trunk. Basically this gave me a total length of about 60 feet of supply trunk with 3 very sharp 90 degree turns just off the AH. The AH is set up to run 2150 CFM, I have 19 - 6" diameter supply lines off of this trunk which I think is okay for 2150 CFM, the problem is the trunk is only 8" X 16". The 3 supply registers closest to the AH actually have negative pressure and is pulling air out of the room.

I can't make the present supply duct bigger so my plan is to add a second 8" X 20" supply duct that will run half the length of the house. I would then take the first 10 6" diameter supply ducts off of the 8" X 16" duct and connect them to the new larger duct. So the old smaller trunk would supply the end of the house furthest away from the air handler. The AH is in the garage, where it comes into the basement it makes a 90 degree turn, I would connect the new supply duct at this 90 and actually turn it into a "T". Since the new duct will be the path of least resistance, because it will be a straight shot down the new duct I want to have a trunk damper added to limit the air into the new duct. I want to also have supply dampers added for all 19 supply lines.

If I could have the system reworked I would, but I just don't have the room to enlarge the present duct, unless I remove the return. Looks like when they built the house they ran the trunks then all the other plumbers and electricians did their work. So there is plumbing, lighting, drains, vents, freon lines...... preventing me from doing so.

Will this work? Any advice would be appreciated. I paid top dollar for this system through a licensed HVAC company. I don't want to dump a lot more money into it and still have problems. Will I have a hard time getting another HVAC contractor to work on this system as screwed up as it is?

Goes to show even top dollar doesn't always guarantee a good job. Why would he have moved the AH from an ideal central location to the end of the house. With your unit located in the centre of the house, the 16x8 duct would have probably done the job. Where your unit is located now makes the duct only half as big as now it has to supply the whole house instead of half each way. The solution you are proposing should work for you, keep in mind you will have to do a decent job of balancing the system. The other consideration is the return air duct. What size is the duct? I would be very concerned about this duct being undersized as well.

probably but not to well
what size is the return duct
keep in mind you can only put out what you take in
if you want to have a true 2000 cfms or more then you need enough return to do it.
as far as the supply duct have an hvac contractor do man d
and install the correct size ducts for the system.
tieing 10 6 in runs into 20 by 8 duct with out reducing the duct down is not a good idea
all you will do is add to the problem and wast monie tring to bandade the system
if you have room to run a trunk line on the other side than it sounds like it would be more cost afective to size it right and do away with the 16 by8 duct completly
again return is just as important and needs to be resized also. you sound like you did some homework on this but a ductulator worked in the old days and still has its place but get a manual d calulation done and do it right for the last time instead of constantly pacthing up old mistakes

4.5 ton system = 1800 CFM
5 ton system = 2000
5.5 ton system = 2200
Info based on 400 cfm per ton.
If you are in a more humid climate, I'd prefer 350 cfm / ton.

Above normal air volumes make heat pumps 'feel cold'

System will not remove as much humidity in the summer time also. Increase humidity, and you will feel warm.
I have seen houses that because of an increase in humidity, they were hot at 68 degrees. I have also seen houses that feel cool at 78 degrees with a lower humidity level. My humidity level actually gets too low in the summer, we end up leaving our stat on 81 (temps been verified by several high quality thermometers. We ask everyone that comes over if they are comfortable and they all say it feels great.)

When you increase the CFM, the air speed inside the ducts also increase. The faster air rushing past your first few openings cause the air to become 'entrained' and make the vents pull in air instead of blowing the air out.
Air entrainment can also occur if the pipe that is connected to the duct is too close and in the throat of an ell.

I'm assuming the CFM setting has been set so high because
the rooms furthest from the unit were not getting enough air flow. Balancing the duct system fixes that prob, not cranking up the fan speed.
Typically this is what happens.
Before:
Vent close to unit 100 cfm / vent furthest from unit 50 cfm
After setting up unit for higher airflow:
Vent close to unit 130 cfm / vent furthest from unit 52 cfm

As you can see.... the problem is amplified.

In your case....
Vent close to unit negative ### cfm hehe

My recommendation.
Have a tech do a 'room by room load calculation'
This will show how much air you need in each room
and also verify the size equipment you have is the
correct size for your house.
Have the tech do a duct sizing calculation per ACCA standards or Ashrae standards (ACCA is most common).
A full duct inspection / evaluation should be made
After the duct modifications are made, then
the tech will need to balance the air flow to obtain
the correct air flow for each room. Balancing should
be done with an accurate air flow meter. You cannot balance by using the back of your hand.

If you havent' had maintenance performed, then now would be a good time to do that.

After everything is complete.
A capacity test should be done and compare the results with manufacturers data.

Thank you for the responses, to address some of the questions and comments:

I think the furnace was orginally okay when it was in the center of the house, there was 8 take-offs from the 8" X 16" on each side fot the furnace/plenum. Like I said I had an addition added which added another 40 feet to the overall length of the house, so basically he just relocated the new system in the new center of the house.

The return duct is probably a problem also, it is 8" X 20", but there are 2 this size, one going to the old house one going to the new addition, the return is not zoned so anytime the AH runs on either zone it pulls air from both zones. This creates another problem though, when the furnace kicks on in one zone all the doors slam shut if they are less than 12" open because I think the zone is blowing more air than is being returned from that zone. The problem with the return is where the duct drops from the ceiling and enters into the bottom of the furnace, this duct is only 14" X 20" and is insulated on the inside which I think makes it even smaller. But with the zoning and individual thermostats both zones don't come on that often, only when it gets very cold out. My thought for this was to have a damper installed, very close to the furnace that would open if both zones were calling for air, hopefully there is something that works manually off of pressure and is not motorized because the motorized dampers I have now have been nothing but problems. Hopefully something that has a filter in it also.

The 2150 cfm speed is what is what the Lennox manual says it should be, there are actually pins to set AC and Heat fan speed seperately.

The problem I have is that I have absolutely no room to enlarge the ducts, this is probably why the contractor didn't, there is a 3" main sewer line that runs just beside the ducts with take-offs, this line cuts across the house which blocks me from extending any new duct that I will have added from extending into the house very far. The basement is finished I would hate to have to drop the ceiling down another 8" to add another supply trunk underneath it.

Should I confront the original contractor and ask him to fix the problems? Does he have any obligations to make it right, 6 years after the fact? I had a lot of problems with him, I paid $11K for the new furnace and heat pump and a little ductwork in 1999. When they added take-offs from the supply duct they just hacked a hole in the top of the supply then put a 6" 90 degree elbow into the hole, there were gaps inches wide open around the supply lines, in addition they didn't support any supplies they added, I could literally just lift them out of the joists, I caught these items before I paid him and made him fix them. The furnace is in a 3 car large unheated garage, in the winter when it's very cold outside the garage stays 55 degrees, this should give you an indication of how many leaks are in the plenum under the insulation. Whenever the system is on heat the freon lines vibrate and sound like a train is coming down the house, the colder it gets the louder the vibration. I could go on & on.

I am a plant manager at a fairly large steel making facility, it is my responsibility to have equipment improvements and maintenance performed on about 800 million dollars worth of equipment and aux. support equipment. Since this is such a large job I have adopted a routine for getting prices and gauranteeing work gets done to my specifications. I engineer, which is just what I'm doing now, then I write a spec. which just describes workmanship and equipment standards, then I get quotes based on the spec. this way the contractor knows exactly what he is responsible for plus it releases him from any performance issues because he is just doing what he has been told to do. I am going to try and handle getting these repairs done in this fashion. I want to tell them exactly what I want done so that I don't end up with even more problems. Are there independent companies, other than the HVAC company itself, that could do the ductwork calculations and possibly look at the site amd make recomendations for placement?

call lenox and get some names of lenox dealers from them
have the system looked over and do the proper losd calculations. basicaly start from the begining and have it done right
lenox will help you out and also give you the names of some good companies in your town

sounds like you might have a plethora of problems that need
to be properly analyzed and addressed.

I want to tell them exactly what I want done so that I don't end up with even more problems

Allow a quality hvac contractor come in and properly diagnose the system and give you their recommendations.
Even though you are an engineer, they will have the technical knowhow and skills to get it done properly.
You would not want a gynocologist to take out your tonsils would you? They are both doctors.. why not?
In the same since, you woulnd't want a cival engineer to
design a motherboard for a computer.

I strongly recommend allowing them to do a complete diagnosis and give you their recommendations.
Then, if you don't like their recommendations.... ask them to do it your way and you will release them of all liability.